The invention relates to an arrangement for reactive power compensation in connection with a power transmission line, the arrangement comprising at least one transformer and at least one reactive power compensator connected to the low-voltage side of the transformer.
The invention further relates to a method for reactive power compensation in connection with a power transmission line, the method comprising compensating for reactive power by means of a reactive power compensator connected to the low-voltage side of at least one transformer.
FIG. 1 shows a prior art arrangement for reactive power compensation in connection with a power transmission line schematically. For the sake of clarity, FIG. 1 shows only one phase, such as phase A, of the power transmission line 1. FIG. 1 also shows a transformer 2 or a main transformer 2, via which the voltage level of the power transmission line 1 can be reduced. On the high-voltage or primary side, the transformer 2 is connected to the power transmission line 1 by a schematically shown connection at a connection point CP1. The arrangement of FIG. 1 also includes, connected to the low-voltage or secondary side of the transformer 2, a voltage busbar 3, of which only one phase, i.e. phase A, is shown schematically. The low-voltage side of the transformer 2 is connected to the voltage busbar 3 by a schematically shown connection at a connection point CP2. The voltage busbar 3 is further connected with a static reactive power compensator 4, which, in the embodiment shown in FIG. 1, comprises a thyristor-controlled reactor (TCR) 5 shown schematically by a triangular symbol and three harmonic filters 6, i.e. filters of harmonic frequencies. The thyristor-controlled reactor 5 comprises a coil and a thyristor switch controlling it, which thyristor switch may comprise up to tens of thyristor levels connected in series and consisting of antiparallel-connected thyristor pairs. The harmonic filter 6, for its part, consists of a coil L and a capacitor C that are dimensioned in a suitable manner and connected in series with one another. The arrangement of FIG. 1 may further comprise an auxiliary transformer 7 connected to the voltage busbar 3 and providing at a substation, where the arrangement of FIG. 1 may possibly be located, a voltage source, which is possibly required by the devices of the substation. By way of example it may be assumed that the power transmission line 1 forms part of the 220 kV high-voltage network, for example, and the voltage busbar 3 forms part of the 20 kV medium-voltage network, for example. In this case, the transformer 2 may be dimensioned, for instance, in such a manner that the power of the transformer 2 may be 150 MVA and reactance 12%, for example.
The arrangement for reactive power compensation in connection with a power transmission line, like the one in FIG. 1, has many different disadvantages. One disadvantage is voltage variation at the voltage busbar 3, which is caused by inductive or capacitive current passing through the transformer reactance. As a result, the voltage of auxiliary electric power supplied by the auxiliary transformer 7 for the substation devices may also vary significantly and cause disturbance in the devices or even break devices. Furthermore, depending on the connection of the reactive power compensator 4 connected to the voltage busbar 3, reactive power effective at the voltage busbar 3 may vary from a value of 150 MVAr of capacitive reactive power to a value of 150 MVAr of inductive reactive power, for instance. To implement a reactive power compensator 4 having such a high power causes significant costs because of the structure and components of the reactive power compensator 4 as well as the installation thereof at the application site. Furthermore, since the aim is to have as low impedance of the transformer 2 as possible to limit the voltage increase induced by capacity current and to lower the transformer price, the result is that, in practice, the transformer 2 cannot limit the short-circuit current and that harmonic overvoltages can easily pass through the transformer 2, which increases the number of the required harmonic filters 6.